Cardiac Pacemaker Mechanisms Flashcards
What is automaticity?
Automaticity implies that cardiac function occurs without any external innervation.
What is the hierarchy of the cardiac pacemakers?
SA node -> Latent Atrial Pacemaker -> AV node/His Bundle -> Bundle Branches -> Purkinje Fibers
What are the ionic mechanisms responsible for the SA node activity?
1) T-type Ca2+ Current
2) Hyperpolarization-activation Inward Current (iF)
3) Deactivation of K+ Current (iK)
4) Inward Na/Ca current activated by SR Ca2+ release
What are the ionic mechanisms responsible for the Purkinje fiber activity?
1) Hyperpolarization-activation Inward Current (iF)
2) Deactivation of K+ Current (iK)
How do changes in threshold potential alter pacemaker activity?
Changing the threshold will make it easier/harder to activate
How do changes in slope of diastolic depolarization alter pacemaker activity?
This is the MAIN change to pacemaker activity and is based on the fact that myocytes do not have a RMP. They only have a slow, smooth transition from maximum diastolic potential to threshold and the slope governs the pace of this transition.
Steeper slope -> increased/more rapid activity
How do changes in maximum diastolic potential alter pacemaker activity?
MDP can be equated to the RMP and so changes will depolarize/hyperpolarize the membrane accordingly.
What is overdrive suppression and how does it affect pacemaker activity?
The SA node normally overrides the intrinsic rate of the other pacemakers in the heart and forces the firing rate to go faster to match its own rate. If the SA node is shut off then the other foci will not be active for sometime before they regain their intrinsic rates.
How does ACh affect pacemaker activity?
ACh stimulates the parasympathetic response in the vagus nerve, which will inhibit some pacemakers, the SA/AV node and latent atrial.
It does so by increasing K+ permeability which makes the maximum diastolic potential more negative and it decreases cAMP which inhibits the L-tyow Ca2+ current.
How does NE affect pacemaker activity?
NE stimulates sympathetic response in all areas of the heart.
It does so by increasing cAMP synths which increase the L-type Ca2+ current and the iF current. It also increases the slope of the diastolic depolarization.
What is sinus arrhythmia?
Normal variability seen in HR cycle length largely caused by vagal nerve activity during respiration.
How does respiration affect sinus arrhythmia?
Inspiration increases HR and expiration decreases HR due to vagal nerve activity.
How do changes in the SA node pacemaker rate show up in the EKG?
Increases in SA node rate show shorter depolarization times and decreases in SA node node show longer repolarizations after the T wave.
What is the T-type Ca current involved in?
It is is turned on with negative potential
and is involved with pacemaker activity. It helps with the depolarization of the heart.
What is the L-type Ca current involved in?
It is involved in the upstroke and plateau of the action potential.
